1. Field of the Invention
The present invention relates to electrophotographic machines, and, more particularly, to a method of controlling laser printhead registration in an electrophotographic machine.
2. Description of the Related Art.
Scanning lasers have significant advantages over light emitting diode (LED) printheads in both monochrome and multi-color printers. These advantages include uniformity of radiant energy across a scan line, uniformity of spot size, a resolution determined by laser diode modulation rather than physical spacings, comparatively low power consumption and resulting low heat generation, optics spaced from the source of toner contamination, and comparatively lower cost.
Although scanning lasers have the above-described advantages, single-pass color electrophotographic printers using scanning laser printheads present registration difficulties not found in printer designs using LED printheads. Initial laser spot position in scan and process directions, line length, skew and bow are all expected variations which affect registration of color planes. Temperature changes of the printhead and its mounting are the major source of change in registration.
A color-to-color image registration problem arises in single-pass laser printers that have more than one laser imaging source. The registration problem is largely attributable to thermally induced changes in scanning laser beam profile and position, laser start-of-scan sensor position, and laser end-of-scan sensor position. Thermally induced changes in the shape and mounting of optical elements, thermally induced changes in the size of the mounting surface, and a relatively long optical path length can contribute to produce line length, skew, and bow scanning spot position changes at the imaging plane of one or more picture elements (pels). The width of a pel is approximately 42.3 xcexcm at 600 dots per inch (dpi). Differences in heating among multiple laser imaging sources then contributes to misregistration of color planes in a composite image. Print quality is generally judged as unacceptable when color plane misregistration exceeds 100 xcexcm.
What is needed in the art is a method of registering a printhead in both the scan and cross-scan directions over a range of printhead operating temperatures.
The present invention provides a method of using temperature sensing in conjunction with optical sensors and electronic feedback to control registration of a scanning laser printhead suited for single-pass EP printing.
The invention comprises, in one form thereof, a method of printing with an electrophotographic machine. A first optical sensor senses a start-of-scan position of a laser beam produced by a scanning laser printhead and transmits a first position signal indicative thereof. A second optical sensor senses an end-of-scan position of the laser beam produced by the scanning laser printhead and transmits a second position signal indicative thereof. A temperature of the scanning laser printhead is measured with a temperature-sensing device. A plurality of positions of each of the first optical sensor and the second optical sensor are empirically determined at each of a plurality of values of the temperature of the scanning laser beam printhead. The modulation or position of the laser beam produced by the scanning laser printhead is adjusted based upon the first position signal, the second position signal, the measured temperature of the laser printhead, and the empirically determined positions of the first optical sensor and the second optical sensor.
The invention comprises, in another form thereof, a method of printing with an electrophotographic machine. A desired scan line length is determined. A plurality of temperatures associated with the electrophotographic machine are measured at respective points in time. A plurality of calibration scan line lengths are empirically determined at a plurality of values of the temperatures associated with the electrophotographic machine. The calibration scan line lengths are used to calculate a scan line length thermal expansion as a function of the temperature associated with the electrophotographic machine. A number of slices in a scan line to be printed is adjusted. The adjusting is dependent upon a current one of the measured temperatures and the scan line length thermal expansion such that a length of the scan line to be printed is substantially equal to the desired scan line length.
An advantage of the present invention is that thermally induced changes in printhead registration can be compensated for.
Another advantage is that thermally induced shifts in the positions of a start-of-scan sensor and an end-of-scan sensor can be compensated for.
Yet another advantage is that the resulting scanning laser printhead has registration performance similar to a modular LED printhead.
A further advantage is that the method of the present invention is applicable to laser printheads in which a single scanning polygon is shared among multiple laser sources to produce multiple scanning laser beams.
The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is a partial, schematic, side view of one embodiment of a laser printer in which the method of the present invention may be used;
FIG. 2 is a top view of one of the laser printheads of FIG. 1;
FIG. 3 is a schematic, side view of the laser printhead of FIG. 2 with an end-of-scan sensor and the corresponding photoconductive drum;
FIG. 4 is a schematic, side view of the laser printhead of FIG. 2 with a start-of-scan sensor and the corresponding photoconductive drum;
FIG. 5 is a plot of the laser scan position of the printhead of FIG. 2 in the scan direction and the cross-scan direction at two different operating temperatures;
FIG. 6 is a plot of the position of the start-of-scan sensor of FIG. 4 at the two different operating temperatures of FIG. 5;
FIG. 7 is a plot of the position of the end-of-scan sensor of FIG. 3 at the two different operating temperatures of FIG. 5;
FIG. 8 is an exemplary plot of the bow in the laser scan position of the printhead of FIG. 2 at four different operating temperatures;
FIG. 9 is a plot of discrete approximations of the four bow characteristics of FIG. 8;
FIG. 10 is an exemplary plot of the bow and skew in the laser scan position of the printhead of FIG. 2 at four different operating temperatures;
FIG. 11 is a plot of discrete approximations of the four bow and skew characteristics of FIG. 10; and
FIG. 12 is a plot of the misregistration in the scan direction of cyan, magenta and yellow versus printhead temperature, without compensation; and
FIG. 13 is a plot of the temperatures of the printheads over the course of a 5000 page run.
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.